1. Field of the Invention
The present invention relates to an ignition timing control apparatus for an internal combustion engine capable of controlling ignition timing so as to always obtain the maximum torque regardless of a kind of fuel and generation of knocking, and regardless of change with time of the engine and fuel to be used.
2. Discussion of Background
As a conventional ignition timing control apparatus for an internal combustion engine, there is one disclosed, for instance, in Japanese Unexamined Patent Publication No. 3175/1984 or Japanese Examined Patent Publication No. 61897/1982.
Various methods of controlling knocking in the engine by using a knocking sensor have been known. There have been two types of method for controlling an engine having a plurality of cylinders. Namely, a type that all cylinders are simultaneously controlled and a type that cylinders are separately and independently controlled.
Generally, there is differences in compression ratio, air-fuel ratio and temperature of combustion chamber wall between cylinders, and an ignition timing in which a knocking occurs is different for each of the cylinders. Accordingly, an excellent output is obtainable by detecting a knocking level for each of the cylinders and by controlling the cylinders so that an ignition timing is delayed for only cylinders in which a knocking takes place.
In the conventional ignition timing control of the cylinders, a basic ignition timing which is common to each of the cylinders is obtained in response to operational conditions of the engine, and a control at the lead angle side is made different from that at the lag angle side. Namely, the control is made so as not to advance timing beyond the basic ignition timing at the lead angle side. With respect to the lag angle side, in order to avoid substantial reduction of torque by unnecessarily lagging the angle of ignition timing in the judgement that knocking takes place by an increased output of a knocking sensor due to scattering in sensitivity, noises and mechanical vibrations in the sensor, a control of knocking level at the lag angle side is conducted in such a manner that a limit value is previously determined in the lag angle region at the point determined by the calculation of [an ignition timing which causes a light knocking condition (the so-called a trace knocking)]-.delta..degree., or at the point of MBT (Minimum advance for Best Torque)-.delta..degree. when the MBT point is at the lag angle side with respect to a trace knocking point. In this case, it is preferable to determine .delta..degree. to be about 5.degree.-6.degree.. In more detail, at first, an ignition is carried out in accordance with the basic ignition timing. When knocking takes place, the ignition timing is delayed so as to correspond to its knocking level to thereby produce a light knocking condition. When there occurs no knocking by such lag angle operation, a lead angle operation is again taken because the ignition timing is excessively delayed. As a result, a light knocking condition is always produced.
If an ignition timing is ahead of the basic ignition timing by such lead angle operation, the ignition timing is adjusted to meet the basic ignition timing. As the basic ignition timing, an ignition timing which provides the maximum torque at a level lower than the light knocking level is determined.
The relation between an ignition timing and a torque generated under a light load condition of engine is indicated by a broken line in FIG. 5. Namely, the ignition timing to obtain the maximum torque (MBT) is deflected toward the lag angle side with respect to the ignition timing to obtain a light knocking level indicated by a point A. However, the relation between an ignition timing and a torque under a heavy load condition is indicated by a solid line in FIG. 5. When gasoline having a low octane is used, the ignition timing to obtain a light knocking level indicated by a point B is deflected toward the lag angle side with respect to the MBT.
When gasoline having a high octane is used, an ignition timing to obtain a light knocking level moves to a point C. There is a case that the MBT is moved to the lag angle side (a high revolution side). Accordingly, the basic ignition timing should be determined at the MBT under a light load condition, and the basic ignition timing should be at the light knocking level or the MBT under a heavy load condition.
In the conventional ignition timing control apparatus having the construction described above, a control of the ignition timing to obtain the maximum torque could not always be obtained in case that the relation among the time of determination of the basic ignition timing, the MBT in actual use and the ignition timing to obtain a light knocking level becomes irregular due to change with time of the engine, or due to a kind of fuel to be used (such as a high octane gasoline, a regular gasoline, alcohol-containing gasoline).
FIG. 6 shows the control of the conventional apparatus described above. In FIG. 6, broken lines represent relations of a knocking level and a torque produced at the time of determination of the basic ignition timing, and solid lines represent relations of a knocking level and a torque produced in actual use affected by the change with time of the engine with respect to an ignition timing.
In a conventional ignition timing control apparatus, since no knocking takes place when the basic ignition timing is just determined, a control is so made as to obtain the MBT for each ignition. However, in the operation having the above-mentioned relation as indicated by the solid line, the ignition timing which provides the maximum torque is moved to a point MBT'. Accordingly, it is clear that the ignition timing as the object of the control is not the ignition timing which provides the maximum torque.
Thus, the conventional ignition timing control apparatus has a problem that it is difficult to control suitably in response to the change of characteristics of the engine, i.e. it is difficult to control ignition timing so as to always obtain the maximum torque.